High-vacuum pumps for high-voltage acceleration tubes



Feb. 16, 1960 R, w, CLOUD ETAL 2,925,504.

HIGH-VACUUM PUMPS FOR HIGH-VOLTAGE ACCELERATION TUBES Filed June l7.1957 2 Sheets-Sheet 1 W T y T 21 I! k 1 I II-J 6 :H W/T- 2 Ill: l I!III! I lllllllll i 1 II Fig. 3

Feb. 16, 1960 R. w. CLOUD ET AL 2,925,504

HIGH-VACUUM PUMPS FOR HIGH-VOLTAGE ACCELERATION TUBES Filed June 17.1957 z Sheets-Sheet 2 HIGH-VACUUM PUMPS FOR HIGH-VOLTAGE ACCELERATIONTUBES Robert W. Cloud, Lexington, and John G. Trump, Winchester, Mass,assignors to High-Voltage Engineering Corporation, Burlington, Mass., acorporation of Massachusetts Application June 17, 1957, Serial No.666,150 ls Claims. (Cl. 313--7) Thisinvention relates to anew type ofhigh vacuum pump which employs an active metal getter such as barium incombination with an activating agent such as ionization. Inparticular,.the invention comprehends a getter-ion pump which forms anintegral part of a particle acceleration tube. In one embodiment of theinvention, heat for the evaporation of a barium getter is provided froma heat source outsidethe accelerationtube tobe evacuated, In. anotherembodiment-of the invention, the amounts. of bariumavailable forevaporation are in excess ofT'IO grams High-vacuum pumps constructedaccording f totheinvention are capable of maintaininga high vacuum insealed-off accelerationtubes of the megavolt class. These tubes, in asense, represent a difi'icult vacuum problem because'they contain notonly the gas ingredients of glass and metal, b'ut'also the'vap'orsof thehydrocarbons used to form strong glass-'to-metal joints betW'eeh'themultiple glass andmetal materials.

Activemetal getters such as evaporatedb'arium do not absorb many organicmaterials so that active metals are not effective. in'maintaining a highvacuum in systems which" are sealed with or contain organic materials.However, the combination 'ofsuch a getter with an activating agent, suchas ionization or heat, is effective in removing fronran enclosed region,not'only the common gettera'ble gases, such as hydrogen, nitrogen,oxygen,.and waterfv'apor, but also the previously non-getterable gasesincluding the noble gases (helium, argon, krypton, etc'.) andceitain'molecules suchas hydrocarbon vapors. In the" case of the ungetterable compoundmolecules, the mechanism probablyinvolves first theirdissociation into the simple getterable ingredients. It,may alsoinvolve,

ast'itd'oes in the case of the noble gases, the activation of themolecules without dissociation. Suchactivation, even though to asub-ionization level, would. tend to render these-molecules"capable'ofattachment to an active metal getter.

Itha's long been known that many substancessuch as barium; potassium,thorium, magnesium, and zirconium are capable of absorbingconsiderablequantities of ordi nary'gasmolecules. The useofbariumandother' gettersis a common device in the manufacture of radio tubes andother"permanently sealed-off systems. These gen-- of getter material,

erally'employ very small quantities generally less than 10' or 100milligrams.

It haslike'wise'beenunderstood for many years that thepr'esen'ceofionization in an evacuated enclosure re sulfsiin the absorption ofresidual gas, includingfeven the noble gases'which arenot absorbedbya'n' active metal getter alone. Such absorption ofresidual gas wasnoted in the old gas-filled X-ray tubesemploying cold cathodes.

These X-raytubes derived their electron current through positive ionbombardment of the cathode, the positive ions beingformed by collisionsof theelectrons in the beam with the residual gas molecules. In suchtubes ssaryto provide a source of gas toprevent the, gradual cleanupofthe tube into a non operative high vacuum condition. Such cleanup'inthe presenceof United States Patent "ice ionization was also noted bymany investigators using closed systems containing ionization gages as ameans of measuring high vacuum pressure.

- It is not believed, however, that the prior art contains descriptionsof getter ion pumping devices particularly. adapted to the evacuation ofsealed-oif'systems wherein the rate of evolution of gas exceeds 10"millimeter-liters per second, whether, due to the large size of thesystem or to gasket material or to organic cements or to other causes.Such systems include, but are not limited to, multiple-electrodehigh-voltage acceleration tubes having;

organic seals, which cannot be heated to conventional outgassingtemperatures, and linear accelerator tubes, which mayfemploy a type ofmetal gaskctseal having a melting point too low' to permit conventionaloutgassing techniques. Thereforewhile in the followingdetaileddescription the invention is described with particular reference to amultiple-electrode high-voltage acceleration" tube, the invention is notlimited thereto, but includes other types of sealed-off systems whereinthe rate of evolution of gas may exceed 10- millimeter-liters persecond. Onesuch system is the so-called tandem particle accelerator,wherein negative ions are accelerated towards a high-voltage terminaland then, after passing through a gaseous-stripper? region, areaccelerated away from the terminal; the stripper region continuouslyevolves gas which must be pumped.

Vacuum devices. may be divided into twocategories: (1)those which arepermanently sealed-01f for the life of the device, and-(2) those whichare continuously pumped to maintain the highvacuum. The use of gettermaterial has generally been confined to permanently sealed- 0E deviceswhich have been very carefully outgassedand. pumped down to a highvacuum prior to seal-off, and the function of the getter has beenconfined to absorbing the residual gases which remain after suchoutgassing, pumping, and seal-oh. These preliminary steps of outgassingand pumping have been thought essential in-connection with sealed-01fdevices, with the getter playiuga subsidiary role. Both processes mustbe carried; outlwith great. care, and are particularly difi'icult in thecase of-systems containing electrodes at high potential, morespecifically, in excess of 100 kilovolts. Such a high voltage device ishard to construct. They must maintain high vacuum over'extended periodsand greatcare is required in their manufacture. Metal for the electrodesoften, must be oxygen free and the electrodes often are outgassedpriorto installation in the tube. assembly. The assembly is subjected, tovigorous heat treatment andoperation' prior to seal-oil using high speeddiffusion.

. pumpsand-traps to collect the emitted gases. Furthermore; in. order tofree such substances of organic vapors, painstaking developments havebeen made of glass and metal which are mechanically compatible, whichcan-:be joined and stillwithstand the high temperature cycling required'in the former ou'tgassing procedure. The introductionof any source ofvapor is scrupulously-avoided, and even so the life of suchtubes isoften limited by gassiness, Which'may result from-inadequate outgassing:procedure.

Devices which cannot be outgassed for one reason or another havealway'sbeen continuously pumped, presumably because the ideaof acontinuous evolution of gas Within the container to be evacuatedsuggests a continuous pumping, system. Accordingly, in the initialdevelopment of high-voltage acceleration tubes, wherein the highenergycharged particles release gases by bombardment of the-solid surfacesWithin the tube, continuous pumping. systems were always used. Inthesealed-01f. acceleration tubes'whichwerelaterdeveloped, it was foundnecessary. tojtakeeXtremely elaborate steps to outgas and pump. down"the acceleration tubes prior to seal-'ofi. Such vigorous treatmentreduces the evolution of gas within these acceleration tubes down to orbelow that prevalent in small radio tubes and accordingly either nogetter is used, or only a small quantitysuch as that used inconventional radio tubes.

The idea of using an adequate amount of getter material coupled with anindependent source of excitation and ionization forthe permanentevacuation of unoutgassed high-voltage acceleration tubes has not beenconsidered prior to this invention. The reasons for this include, inaddition to those hereinbefore mentioned, the undesirable effectsresulting when the getter material is exposed either to high energycharged particles or to high voltage gradients. Moreover, if this ideahad been proposed earlier, it would have been rejected as unworkable. Ithas been well known in the art that if the cases normally entrained inan unoutgassing cubic centimeter of copper are released, the quantity ofgas thus released will amount to hundreds of cubic centimeters atatmospheric pressure. In accordance with the invention, a sufficientlylarge quantity of getter material is used to absorb gases during thelife of the evacuated device, and the getter material is shielded fromthe high energy charged particles and from intense electric fields.Moreover, the getter material is not permitted to become overheated,whereby the tendency of the getter to migrate is minimized. In apreferred embodiment of the invention, the getter material is vaporizedso as to form an extended surface by a heat source external to thechamber to be evacuated.

r In accordance with the invention, it is possible to combine materialswithout excessive regard for their outgassing properties. Glass andmaterials may be chosen for their electrical characteristics and may bejoined with an organic seal or low-melting-point solder or metal gasket.The tube need not be subjected to any temperature cycling or operatedprior to seal-off. Before the pump of the invention is used, the tubeneed be evacuated only with a mechanical pump to a pressure of 10-millimeters of mercury or thereabouts. Even this use of a mechanicalforepump may be eliminated, since the pump of the invention is capableof operating even at atmospheric pressure. Moreover, the invention isapplicable to tubes characterized by very high gradient requirementssuch as acceleration tubes, operating at a total voltage of the order ofseveral million volts and with electrical gradients lengthwise of 5.00kilovolts per foot or more. The invention may best be understood fromthe detailed description thereof having reference to the accompanyingdrawing in which:

Fig. ,1 is a side view, partly in longitudinal central section andpartly broken away, of a multiple-electrode high-voltage accelerationtube incorporating a high vacuum pump in accordance with the invention;

. Fig. 2 is a perspective view of a longitudinal central section of aportion of the acceleration tube of Fig. 1; and

Fig. 3 is a side view similar to that of Fig. 1 but showing theevaporation device by means of which the getter is evaporated inaccordance with the invention.

Referring to the drawings, there is shown in Figs. 1-3 an accelerationtube of the general type disclosed in United States Patent No. 2,460,201to Trump and Cloud and comprising a multiplicity of alternatinginsulating rings 1 and apertured electrode disks 2 terminating at oneend in a cathode 3 and at the other end in an extended anode 4. V Suchan acceleration tube may be adapted to accelerate various types ofcharged particles, such as positive ions, negative ions or electrons,and merely by way of example the acceleration tube shown in Fig. 1,in anX-ray tube wherein electrons emitted at the cathode 3 are acceleratedonto a gold target 5 for the production of X-rays. The tube is assembledin a vacuum-tight manner and is then evacuated through a-copper tube 6in the cathode 3 of the acceleration tube by a mechanical pump a issues(not shown). For example, the tube may be pumped down in this manner for3 days. The tube is then sealed ofi' by pinching the copper tube 6 and 7so as to form a cold-welded seal. The tube is then evacuated to a highvacuum by means of the getter ion pump which has been made integral withthe acceleration tube in accordance with the invention and which willnow be described. in detail.

The getter ion pump comprises in combination an ionization gauge 8 and agetter chamber 9, both of which form part of the extended anode 4 andwhich are therefore at ground potential except as noted hereinafter; Theionization gauge 8 operates on well-known principles whereby electronsare accelerated towards an electrode, but are hindered from reachingsaid electrode by the small dimensions of the electrode and by thedeflecting action of a magnetic field. In the drawing, the electrodetoward which the electrons are accelerated comprises a filamentary ring10 which is supported by four'short rods 11 of .030-inch stainless steelwire attached. to an apertured electrode disk 12. This filamentary ring10 is supported in the magnetic field which exists between two ringmagnets 13. Some mild steel pieces 14 provide a path for the return fluxand shield the charged particles being accelerated by the accelerationtube from the mag netic field. The magnets 13 are at ground potentialand a positive potential of 2800 volts is applied to the fila mentaryring 10 by a suitable power supply 15. Eleo-' trons between the ringmagnets 13, whether produced by field emission from the surface of themagnets 13 or by. ionization due to cosmic rays or otherwise, 'areaccelerated towards the filamentary ring 10; but the magnetic field,which may be of the order of 10 gausses, causing the electrons to travelin helical paths so that they do not strike the filamentary ring 10until after they have traveled great distances. As a result, the deviceacts. as an ionization gauge. That is to say, the device ionizes the gasin its vicinity, and the resultant current between the magnets 13 andthe filamentary ring 10 is a measure of the'gas pressure within theacceleration tube and may be read on the meter 16.

Below the ionization gauge 8 is provided a grounded getter chamber 9.Pieces of barium 17 are placedvin the bottom of the chamber 9, and thetop is enclosed by an aluminum plate 18 which is provided with a seriesof apertures 19 and to which is attached one or more baflles 20. Eachbafiie 20 may comprise, for example, a SO-mesh stainless steel screenwhich is spot welded in tubular. form. The upper wall 21 of the chamber9 is surrounded by a thick tube 22 of mild steel which is permanentlywelded to the chamber 9. The lower wall 23 vof the. chamber 9 is thinnerthan the upper wall 21'and the mild steel ring 24 which surrounds it isremoved during evaporation of the getter. The purpose of the mild steelmembers 22 and 24 is to shield the charged particles being acceleratedby the acceleration tube from external magnetic fields such as thoseproduced by the drive motor of an electrostatic accelerator in which thetube might. be placed. a

, Referring now to Fig. 3, during evaporation of the barium getter, thelower wall 23 of the chamber9 is surrounded by an oven 25, while theupper wall 21 of the chamber 9 is surrounded by a water-cooled jacket26. The barium 17 is heated to evaporation temperatures, such as 820centig rade, in the oven and the barium vapor is condensed principallyon the inner surface of the upper wall 21 of the chamber 9 but also onthe baflles 20. Surfaces on which the barium getter is deposited areshielded from later being struck by the beam of charged particles byproviding cylindrical tubular members 27 through which said beamtravels. It will now be seen that the apertures 19 in the top plate 18are necessary in order that any gas within the acceleration tube maytraveltherethrough and be trapped by the getter;

As stated hereinbefore, an importantfeatu reof;

invention is the provision of heat for the evaporation of the getterfrom a heat source outside i the acceleration tube tobe evacuated. Inaccordance with this; aspect of the invention, it is only necessarythat-the actual heat source be external to the evacuated region and notexposed thereto. Thus, for example, the external heat source of theinvention may comprise a hollow metal tube which is introduced into theevacuated .region through a vacuum-tightseal and within which a heatingelements provided. The heat source is thus external to the evacuatedregion and-is separated therefrom by the hollow metal tube, which isgrounded and vacuum tight.

In our work wehave employed an active metal getter, preferablybarium, incombination with ionization to pump on the complex gaseousingredientsiofhigh voltage acceleration tubes. We have preferentially used barium asthe active metal getter and have included relatively large amounts ofbarium in these pumps in order to give them extended pumping timesranging into years. Even a small pump of this type for example iscommonly given between and 100 grams of barium, and pumps are now beingdesigned which contemplate an initial loading of pounds of barium.

In one embodiment of the invention the pump is of the intermittent type.The barium is evaporated onto a cold surface which subsequently absorbsover an extended period of time the gas molecules which come in contactwith it. On a typical acceleration tube of the megavolt class, a singleevaporation, for example, may subsequently maintain a highvoltage-insulating vacuum within the tube for a period rangingfrom 1month to 1 year. This pump, however, is capable of repeated evaporationcycles whenever further fresh gettering surfaces become necessary.

The invention also comprehends a container loaded with getter-coatedsurfaces and provided with a source of ionization. Such containers couldbe bolted on any system requiring the maintenance of a high vacuum andthen connected to it by opening a valve. This would in effect be agetter-ionization pump in which the evaporation has been accomplished inadvance.

Having thus described the method of the invention, together with apreferred embodiment of apparatus for carrying out the method, it is tobe understood that although specific terms are employed, they are usedin a generic and descriptive sense and not for purposes of limitation,the scope of the invention being set forth in the following claims.

We claim:

1. In combination with a sealed-off system containing electrodes at highpotential, means for exciting and ionizing the residual gas within saidsealed-off system, a metallic getter within said sealed-off system, andmeans for shielding said excitation-ionization means and said getterfrom the electric fields associated with said high potential.

2. In combination with a sealed-off system containing electrodes at highpotential, means for exciting and ionizing the residual gas within saidseal-oil system, a metallic getter within said sealed-off system, and aheat source external to said sealed-off system and adapted to evaporatesaid getter.

3. In combination with a sealed-off system containing electrodes at highpotential, means for exciting and ionizing the residual gas within saidsealed-off system, an amount of barium within said sealed-elf system atleast of the order of 10 grams, and a heat source external to saidsealed-off system and adapted to evaporate said barium periodically.

4. In combination with apparatus for the acceleration of chargedparticles including a tube traversed by a stream of charged particles,the construction of said tube being such that the rate of evolution ofgas therein may exceed 10 millimeter-liters-persecond, agetter-ionization pump integral with-said--tube.

, 5. In combination with apparatus for the acceleration of 'chargedparticles including a tube traversed by a streamof chargedparticles, theconstruction of said tube within said tube having an extensive surfacearea, and

means for shielding said ionization means and said getter from the beamof charged particles accelerated by said tube during operation thereof.

7. Apparatus in accordance with claim 6 wherein said getter comprisesbarium.

8. Apparatus in accordance with claim 7 wherein said getter comprisesbarium in an amount at least of the order of 10 grams.

9. In combination with a multiple-electrode acceleration tube comprisinga series of alternating insulating rings and apertured electrodescemented by a synthetic resin and closed off at one end by an end pieceand at the other end by a tube extension, a getter-ion pump integralwith said acceleration tube.

10. In combination with a multiple-electrode acceleration tubecomprising a series of alternating insulating rings and aperturedelectrodes closed olf at one end by an end piece and at the other end bya grounded tube extension, a getter-ion pump integral within saidgrounded tube extension.

11. A multiple-electrode acceleration tube comprising in combination aseries of alternating insulating rings and apertured electrodes cementedby a synthetic resin and closed ofi at one end by a cathode and at theother end by an anode extension, a pair of axially aligned ring magnetssupported within said anode extension, a filamentary ring supportedbetween said ring magnets in axial alignment therewith, means forimparting a positive voltage to said filamentary ring with respect tosaid ring magnets, a getter within said anode extension having anextensive surface area, and at least one tubular shield supportedaxially within said anode extension so as to shield said ionizationmeans and said getter from the beam of electrons accelerated by saidtube during operation thereof.

12. A method of assembling a high-vacuum sealed-off system withoutoutgassing the same, which method comprises the following steps:assembling a system and hermetically sealing the same with the exceptionof an aperture for pumping, said system having therein an amount ofmetallic getter at least of the order of 10 grams. and ionization means,said assembly being performed Without outgassing so that the rate ofevolution of gas within said system may exceed 10 millimeter-fliers persecond, evacuating said system to fore-vacuum pressure through saidaperture, hermetically sealing said aperture so as to seal off saidsystem, evaporating said metallic getter by means of a heat sourceexternal to said sealedotf system, and cooling an extensive surface areawithin said sealed-off system so that the evaporated getter condensesthereon.

13. A method of assembling an acceleration tube which method comprisesthe following steps: cementing together a multiplicity of alternatinginsulating rings and apertured electrodes; sealing to the end thereof anend-piece having a small tube therethrough and a tube extension,respectively, said tube extension having therein a large amount ofbarium and ionization means; evacuating said accelera- '7 tion tube tofore-vacuum pressure through said small tube in said end-piece;pinchingofi said small tube so as to seal ofli said acceleration tube;evaporating said barium by means of a heat source external to saidacceleration tube; and cooling an extensive surface area within saidtube extension so that the evaporated barium condenses thereon.

14. A method in accordance with claim 13 wherein the amount of bariumevaporated is at least of the order of 10 grams.

15. In combination with a sealed-off system for the acceleration ofcharged particles whose envelope includes materials which do not permitthe application of temperatures substantially above 200 C. foroutgassing purposes, a getter-ionization pump integral with'saidsystern.

' References Cited in the file of this patent UNITED STATES PATENTS2,332,428 Atlee et a1. Oct. 19, 1943 2,540,647 Bienfait Feb. 6, 19512,636,664 Hertzler Apr. 28, 1953 2,640,948 Burrill June 2, 19532,656,489 Ford Oct. 20, 1953 2,796,555 Connor June 18, 1957 Yoder June10, 1958 OTHER REFERENCES Evapor-Ion Pump, by H. G. Herd, R. H. Davis,A. S. Divatis and D. Saxon, Physical Review, 2nd series,

15 vol. 89, No. 4, page 897, February 13, 1953.

